1. Field of the Invention
The present invention relates generally to metal chelation and more specifically to the selective and reversible removal or concentration of a target metal ion.
2. Description of the Background Art
Most commercial methods for removing undesirable metal ions are presently done using reverse osmosis (RO) and/or ion-exchange columns. However, RO is expensive, requires several precleaning stages (that is, it cannot be used in a cost-effective way to treat heavily contaminated waters), and is very susceptible to fouling. Ion-exchange columns can be used for many applications where selectivity and the exchange of one kind of ion for another are not critical factors, but could present problems in other situations. The materials for these columns also do not have a very high binding constant.
In order to get around the above issues, strong chelating agents, such as crown ethers or cyclic amines, can be used. Chelating agents have the advantage of being selective for transition metal ions over most group IA and IIA metal ions and of possessing very high binding (formation) constants, both of which are useful properties for selectively taking up metal ions in a mixed ion system.
These crown ethers or cyclic amines achieve their top formation constants and selectivity through the preorganization of their electron-donors into conformations that are optimal for particular metal ion sizes and coordination geometries. However, the high expense and general unavailability of these materials on a commercial scale preclude their common usage. On the other hand, acyclic chelating agents are common and very low cost compared to the macrocycles. Their disadvantages are their lower binding constants and selectivities in comparison to those of the macrocycles.
Ideally, acyclic chelating agents could be locked into an optimal geometry such that a target metal ion, and no other ion, could fit into that nanoscale receptacle. Current knowledge of the required conformations and thermodynamics needed for optimization is inadequate for designing sites having those optimal geometries. Additionally, the cost of such a "chelation by design" approach would be prohibitive and would eliminate the cost advantage possessed by acyclic chelating agents.